Not a minor point at all. I think sexual fluidity is one of the most relevant aspects of sexuality and I would love to see it measured. Labels like "Gay" and "Bi" are way too simplistic.
The medical terminology is better, its often "Men who have sex with men" or something similar that removes the need to understand the subjects mental state and simply focus on the important clinical data.
That's better, but doesn't separate men who have sex with manly men from those who only have sex with traps. It also excludes those who don't have sex at all for one reason or another.